It will not be supported on Yosemite. Developers have had many many years to update their software for newer compilers. If you still need gcc 4.2 for some reason, please file a bug with llvm.org indicating what the issues are with clang that you need fixed.

On fink, I have solved our llvm-gcc42 build problems with the attached patch. The first part of the patch is a back port of the upstream fix for the __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ support...

There is one additional section to the patch that modifies the sed in llvmgcc42-2336.11/llvmCore/Makefile.rules that sets DARWIN_VERSION and DARWIN_MAJVERS. I suspect that change is only valid for 10.10 and later so you will only want to apply the patch for those releases.